Heretofore, a DCCT (DC Current Transformer) of a magnetic modulation type is known, for example, as a beam current meter.
In this respect, however, the lower limit for measuring an electric current is in an order of several μA in a conventional DCCT of the magnetic modulation type, so that there is such a problem that a faint beam current of around several nA cannot be measured.
In order to solve such problem as described above, a beam current meter composed of a SQUID (Superconducting Quantum Interference Device) used for measuring a magnetic field produced by a brain or a heart and a magnetic shield of a superconductor operated at a liquid helium temperature has been developed by German GSI (Gesellshaft fur Schwerionenforschung), Institute for Nuclear Study (INS), University of Tokyo (now High Energy Accelerator Research Organization, KEK), or Research Center for Nuclear Physics, Osaka University as a beam current meter which can measure a faint beam current of an around several nA order (see non-patent literary documents 1 to 5).
When a beam current meter with which a faint beam current of around several nA can be measured is compared with a conventional beam current meter with which a beam current of an order of several μA is measured, the former beam current meter has 1000 times higher sensitivity than that of the latter beam current meter. In a comparison of specific magnetic fields, earth magnetism is 10−5 T, while cerebric magnetic field is 10−15 T wherein a point of 20 cm apart from the center of a magnetic field produced by beam of 1 nA is 10−15 T, so that a beam current meter which may measure a faint beam current of around several nA must measure a very faint magnetic field.
FIGS. 1 and 2 show schematic structural constitutions of a conventional beam current meter composed of the above-described SQUID and a superconductor magnetic shield which operates at liquid helium temperature. Namely, FIGS. 1 and 2 illustrate only a constitution of its machine construction to help understanding of the present invention, so that a variety of electrical connecting conditions, an electrical conductive state, and a detecting means for temperature and the like are not illustrated and they are omitted. FIG. 1 is a sectional view taken along the line A-A of FIG. 2, while FIG. 2 is a sectional view taken along the line B-B of FIG. 1.
In the description of the present specification and in the accompanying drawings, the same or corresponding components as or to other components are represented by the same reference character, whereby an overlapped description as to their constitutions and functions is omitted.
In FIGS. 1 and 2, reference numeral 1 designates a superconductor beam current sensor, reference character 2 designates a superconductor magnetic shield, reference character 3 designates a SQUID, reference character 4 designates a cooling medium tank, reference character 5 designates a vacuum vessel, reference character 6 designates an upper flange, reference character 7 designates a beam duct, reference character 8 designates a trestle, reference character 9 designates liquid helium being a cooling medium, reference character 10 designates a vacuum area in the vacuum vessel 5, and reference character 11 designates an atmospheric air area outside the vacuum vessel 5.
More specifically, the vacuum vessel 5 is constituted in such that a side of the upper surface 5a is closed by the upper flange 6, bores 5c for constituting beam ducts 7 are defined on a circumferential wall surface of the vacuum vessel at positions opposed to each other, and a side of the bottom surface 5b is supported on the trestle.
In the beam current meter, it is arranged in such that a beam is input from either of the bores 5c constituting either of the beam ducts 7, and the beam is output from the other bore 5c constituting the other beam duct 7.
In the vacuum vessel 5, a cylindrical beam current sensor 1 is arranged in such a manner that the beam incoming into the vacuum vessel 5 is guided to pass through a bore of the vacuum vessel, and the SQUID 3 is disposed on a side of the upper surface of the beam current sensor 1. Moreover, the cylindrical superconductor magnetic shield 2 is arranged so as to surround the external diameter side of the beam current sensor 1 in such a manner that the SQUID 3 is positioned between the beam current sensor 1 and the magnetic shield.
These above-described beam current sensor 1, SQUID 3, and magnetic shield 2 are arranged in the cooling medium tank 4 of a doughnut shape wherein the above-described respective components are arranged in such that a beam is guided to pass through a hollow region in the internal diameter side of the doughnut-shaped cooling medium tank 4.
The cooling medium tank 4 is filled with liquid helium being a cooling medium, whereby the beam current sensor 1, the SQUID 3, and the magnetic shield 2 disposed in the cooling medium tank are cooled to the liquid helium temperature.
In the above described constitution, the inside of the vacuum vessel 5 is maintained at 1×10−4 Pa by means of vacuum equipment, and a beam is passed through the beam current meter. Namely, a beam is passed through the beam current meter in such a manner that the beam is input from either of the bores 5cconstituting either of the beam ducts 7 and output from the other bore 5c constituting the other beam duct 7, whereby a beam current of the beam is measured.
Although such measuring principle of a beam current meter wherein the beam current sensor 1 and the SQUID 3 are used is a well-known technology, it will be simply described by referring to FIG. 3 for easy understanding of the invention according to this application.
FIG. 3 is a perspective explanatory view of a schematic constitution showing the beam current sensor 1 to which the SQUID 3 is attached.
The beam current sensor 1 is formed by winding circumferentially a linear insulating material around a surface of the external diameter side of a superconductor circumferential wall surface (in headband-like state) with leaving only a part of a region (bridge portion). The above-described insulating material is disposed circumferentially at the central position in the axial direction of the beam current sensor 1. Moreover, the SQUID 3 is disposed on the above-described bridge portion.
When a beam passes through a space on the internal diameter side of the beam current sensor 1, shield current flows on the surface of the superconductor based on Meissner effect. The shield current flows only through the bridge portion, whereby a magnetic field in an azimuthal direction is generated as a result of passage of an electric current.
Since the SQUID 3 is provided on the bridge portion, a magnetic field produced in the bridge portion as a result of the passage of an electric current may be measured at high sensitivity, so that when the magnetic field measured at high sensitivity is converted into an electric current value, a beam current can be measured in nondestructive and with a high degree of accuracy.
Namely, when the bridge portion is formed on the surface of a cylindrical superconductor, it becomes possible to concentrate efficiently a shield current.
In order to measure with a good SN ratio such magnetic field generated in an azimuthal direction on the bridge portion, it is preferred to use a SQUID gradiometer as the SQUID 3.
This is because the SQUID gradiometer contains right and left input coils for detecting a magnetic field as shown in FIG. 4. In this case, if there is a common mode noise magnetic field wherein magnitudes and directions of external noise magnetic fields are quite same with each other in the case when external noise magnetic fluxes are going to enter the right and left input coils, the external noise fluxes are completely cancelled. On one hand, a magnetic field produced in the bridge portion as a result of passage of a beam is an opposite phase magnetic field wherein magnitudes are the same, but directions are opposite to each other as described above. Thus, when the SQUID gradiometer is compared with a SQUID magnetometer wherein one input coil is usually used, the SQUID gradiometer can detect data at two times higher sensitivity than that of the SQUID magnetometer.
When a SQUID gradiometer is used as the SQUID 3, an external noise magnetic field can be remarkably reduced, a limit of sensitivity due to a conventional DCCT of magnetic modulation type comes to be improved remarkably as a result of an application of such superconductive technology.
Incidentally, although the SQUID 3 is used for high sensitization in the above-described beam current meter, signals required actually cannot be detected in the case where such signals get lost in noises from the outside, even if a how high sensitive sensor is used. For this reason, the superconductor magnetic shield 2 is arranged in the above-described conventional beam current meter.
Namely, when a magnetic field is applied to a superconductive material from the outside, an electric current flows on a surface of the superconductive material as a characteristic of superconductivity in such that the magnetic field is canceled due to Meissner effect. It has been widely known that a strong magnetic shield can be realized by applying the above-described effect.
Moreover, although a conventional magnetic shield surrounding a metal of a high magnetic permeability is effective in a high frequency magnetic field, the effect decreases significantly in a low frequency magnetic field. In case of a superconductor magnetic shield, there is a remarkable advantage to the effect that its efficiency of shield does not depend on a frequency.
The magnetic shield 2 may be prepared by calcining a bismuth-base superconductive material with a thickness of 300 microns on a cylindrical ceramics made from magnesium oxide having a purity of 99.9% or more. For the preparation of such magnetic shield, about four weeks of processes such as calcination, and compression are required.
For measuring the magnetic shield 2 prepared as described above, an X-Y stage driven by a stepping motor provided with a Helmholtz coil and a SQUID system for measuring magnetic field are fabricated, and measurement is conducted. Namely, as shown in FIG. 5, an external magnetic field is generated over the magnetic shield 2 by means of the Helmholtz coil, and a magnetic field probe provided with the SQUID 3 is driven between the magnetic shield 2 and the beam current sensor 1, whereby an attenuation factor of the magnetic field is measured.
FIG. 6 shows results of measuring attenuation factors in a magnetic field wherein a magnetic field of 3.5 μT is applied parallely to the magnetic shield 2 in 1 Hz cycle. In this case, a position 0 mm of the magnetic shield 2 represents the cylindrical center of the magnetic shield 2. An attenuation factor S(z) is defined by the following equation:S(z)=B(z)/B0wherein B0 represents a magnetic field produced by the Helmholtz coil, and B(z) represents a magnetic field at a position z of the magnetic probe in the magnetic shield 2.
From the measuring results, such result that the attenuation factor is 3×10−4 at the center of the magnetic shield 2 was obtained in the case when a magnetic field is applied parallely to the magnetic shield 2.
However, a beam current meter wherein a superconductor operated at liquid helium temperature is used involves the following problems, because liquid helium is used as a cooling medium as described above. The specific problems are in that since liquid helium is used as a cooling medium, a cooling mechanism therefor becomes complicated, and in addition a cost therefor becomes expensive, because the liquid helium itself being a coolant (cooling medium) is not an inexpensive material.
Furthermore, there is another problem in that when liquid helium is replenished in case of using the liquid helium as a cooling medium, several hours are required until a SQUID is operated stably, so that such replenishing operation requires much labor hour and time.
In addition, there is a further problem in that beam current measurement with high sensitivity cannot be implemented in the above-described conventional magnetic shield in view of the attenuation factors in the measured results shown in FIG. 6 as described above.    Non-patent literary document 1: “A Cryodevice for induction monitoring of DC electron or ion beams with nano-ampere resolution”, K. Grohmann, et al., Superconducting Quantum Interference Devices and Their Applications, 1977, p.311    Non-patent literary document 2: “SQUID” based beam current meter”, IEEE Trans. on Magnetics, Vol. MAG-21, No. 2, 1985, p.997    Non-patent literary document 3: “A Cryogenic current comparator for the absolute measurement of nA beams”, AIP Cof. Proc. 451 (Beam Instrumentation Workshop), 1998, p.163    Non-patent literary document 4: “Design and performance of an HTS current comparator for charged particle-beam measurements”, L. Hao et al., IEEE Trans. on Appl. Supercond. (ASC2000), Vol. 11, No. 1, 201-3, p.635    Non-patent literary document 5: “TYOZOU RINNGU NIOKERU BIIMU DENRYUU NO KOUKANNDO SOKUTEI (High sensitive measurement of beam current in storage ring)”, Tetsumi Tanabe, Megumi Shinada, Journal of Physical Society of Japan, Vol. 54, No. 1, 1999, p.34